Liquid fuel rocket engines require a supply of propellant at 300-8000 psi at a high flow rate and at a steady pressure. The propellant may be supplied from a tank at the required pressure or a pump may be used to raise the pressure of a propellant stored at low pressure. If a pump is used, it must be of minimum weight and have high reliability.
The most important factors for rocket performance are the type of propellant used and the empty or burnout mass of the rocket which contains a given amount of propellant. For any given propellant, the performance of a rocket depends of the weight of the propellant tanks, the weight of the engine and the weight of the pumps, if required. Each of these components must be as light as possible for optimum performance. Typically, there are two options for supplying propellant to the rocket engine, one way is to pressurize the tanks and the other way is to use a turbopump. Pressurizing the tanks, however, requires heavy tanks made from exotic and expensive high strength materials which reduce rocket performance because of their weight and increase the costs. If turbopumps are utilized, complexity of the rocket increases, and thus the reliability is decreased, and the costs are increased. Most all large liquid rockets from the V2 to the Atlas V's use a turbopump to supply fuel to the engine. In these rockets, the turbopump is one of the most complex components of the rocket system. Turbopumps typically rotate at 30,000-100,000 RPM to develop the power required for the rocket.
The cost of turbopumps reflect the large amount of engineering design and testing efforts that are required for turbopumps. Also, the manufacturing of turbopumps require precision machining of the exotic alloys. The failure of a turbopump usually results in an explosion, which can be disastrous to the rocket if the pump is filled with liquid oxygen. All of these items drive up the cost of a turbopumps. In addition, turbopumps cannot be run to the point of the fuel tank being empty due to problems with overspeeding and cavitation, both of which may also cause catastrophic failure. Therefore, a substantial amount of fuel must be left in the tank of the rocket that uses a turbopump, which increases the burnout weight of the rocket. A turbopump also requires a few seconds to startup, and during the startup time the rocket performance is not optimal. Furthermore, a rocket system which uses a turbopump generally burns a significant portion of the propellant in the gas generator which drives the turbopump, thereby decreasing the performance of the rocket vehicle.